chore: add functional test mr.merge() with long commit message

Functional test to show that
#1452 is fixed.

Added a functional test to ensure that we can use large commit message
(10_000+ bytes) in mr.merge()

Related to: #1452

tools/functional/api/test_merge_requests.py

@@ -154,3 +154,62 @@ def test_merge_request_should_remove_source_branch(
 
     mr = project.mergerequests.get(mr.iid)
     assert mr.merged_at is not None  # Now is merged
+
+
+def test_merge_request_large_commit_message(
+    project: gitlab.v4.objects.Project, wait_for_sidekiq
+):
+    """Test to ensure https://github.com/python-gitlab/python-gitlab/issues/1452
+    is fixed"""
+    source_branch = "large_commit_message"
+    project.branches.create({"branch": source_branch, "ref": "master"})
+
+    # NOTE(jlvillal): Must create a commit in the new branch before we can
+    # create an MR that will work.
+    project.files.create(
+        {
+            "file_path": f"README.{source_branch}",
+            "branch": source_branch,
+            "content": "Initial content",
+            "commit_message": "New commit in new branch",
+        }
+    )
+
+    mr = project.mergerequests.create(
+        {
+            "source_branch": source_branch,
+            "target_branch": "master",
+            "title": "Large Commit Message",
+            "remove_source_branch": True,
+        }
+    )
+
+    result = wait_for_sidekiq(timeout=60)
+    assert result is True, "sidekiq process should have terminated but did not"
+
+    mr_iid = mr.iid
+    for _ in range(60):
+        mr = project.mergerequests.get(mr_iid)
+        if mr.merge_status != "checking":
+            break
+        time.sleep(0.5)
+    assert mr.merge_status != "checking"
+
+    # Ensure we can get the MR branch
+    project.branches.get(source_branch)
+
+    commit_message = "large_message\r\n" * 1_000
+    assert len(commit_message) > 10_000
+    assert mr.merged_at is None  # Not yet merged
+
+    mr.merge(merge_commit_message=commit_message, should_remove_source_branch=True)
+
+    result = wait_for_sidekiq(timeout=60)
+    assert result is True, "sidekiq process should have terminated but did not"
+
+    # Ensure we can NOT get the MR branch
+    with pytest.raises(gitlab.exceptions.GitlabGetError):
+        project.branches.get(source_branch)
+
+    mr = project.mergerequests.get(mr.iid)
+    assert mr.merged_at is not None  # Now is merged